(#) Unknown nullness !!! WARNING: Unknown nullness This is a warning. Id : `UnknownNullness` Summary : Unknown nullness Note : **This issue is disabled by default**; use `--enable UnknownNullness` Severity : Warning Category : Interoperability: Kotlin Interoperability Platform : Any Vendor : Android Open Source Project Feedback : https://issuetracker.google.com/issues/new?component=192708 Since : 3.2.0 (September 2018) Affects : Kotlin and Java files Editing : This check runs on the fly in the IDE editor See : https://developer.android.com/kotlin/interop#nullability_annotations Implementation : [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/InteroperabilityDetector.kt) Tests : [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/InteroperabilityDetectorTest.kt) To improve referencing this code from Kotlin, consider adding explicit nullness information here with either `@NonNull` or `@Nullable`. !!! Tip This lint check has an associated quickfix available in the IDE. (##) Options You can configure this lint checks using the following options: (###) ignore-deprecated Whether to ignore classes and members that have been annotated with `@Deprecated`. Normally this lint check will flag all unannotated elements, but by setting this option to `true` it will skip any deprecated elements. Default is false. Example `lint.xml`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~xml linenumbers <lint> <issue id="UnknownNullness"> <option name="ignore-deprecated" value="false" /> </issue> </lint> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/test/pkg/Test.java:10:Warning: Unknown nullability; explicitly declare as @Nullable or @NonNull to improve Kotlin interoperability; see https://developer.android.com/kotlin/interop#nullability_annotations [UnknownNullness] public Object error1(Integer error2, int[] error3) { return null; } ----- src/test/pkg/Test.java:14:Warning: Unknown nullability; explicitly declare as @Nullable or @NonNull to improve Kotlin interoperability; see https://developer.android.com/kotlin/interop#nullability_annotations [UnknownNullness] public Float error4; ----- src/test/pkg/Test.java:16:Warning: Unknown nullability; explicitly declare as @Nullable or @NonNull to improve Kotlin interoperability; see https://developer.android.com/kotlin/interop#nullability_annotations [UnknownNullness] public Float error5; ----- src/test/pkg/Test.java:18:Warning: Unknown nullability; explicitly declare as @Nullable or @NonNull to improve Kotlin interoperability; see https://developer.android.com/kotlin/interop#nullability_annotations [UnknownNullness] public Object error6() { return null; } ------ src/test/pkg/Test.java:19:Warning: Unknown nullability; explicitly declare as @Nullable or @NonNull to improve Kotlin interoperability; see https://developer.android.com/kotlin/interop#nullability_annotations [UnknownNullness] protected Float error7; ----- src/test/pkg/Test.java:34:Warning: Unknown nullability; explicitly declare as @Nullable or @NonNull to improve Kotlin interoperability; see https://developer.android.com/kotlin/interop#nullability_annotations [UnknownNullness] public Float error8; ----- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the source file referenced above: `src/test/pkg/Test.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; import androidx.annotation.NonNull; import androidx.annotation.Nullable; @SuppressWarnings({"ClassNameDiffersFromFileName", "MethodMayBeStatic"}) public class Test { public void ok(int x, float y, boolean z) { } @Nullable public Object ok2(@NonNull Integer i, @NonNull int[] array) { return null; } private Object ok3(Integer i) { return null; } public Object error1(Integer error2, int[] error3) { return null; } @NonNull public Float ok4 = 5; @NonNull protected Float ok5 = 5; private Float ok6 = 5; public Float error4; /** Field comment */ public Float error5; /** Method comment */ public Object error6() { return null; } protected Float error7; // Don't flag public methods and fields in non-public classes or // in anonymous inner classes @SuppressWarnings("ResultOfObjectAllocationIgnored") class Inner { public void ok(Integer i) { new Runnable() { @Override public void run() { } public void ok2(Integer i) { } }; } } @Deprecated public Float error8; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can also visit the [source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/InteroperabilityDetectorTest.kt) for the unit tests for this check to see additional scenarios. The above example was automatically extracted from the first unit test found for this lint check, `InteroperabilityDetector.testNullness`. To report a problem with this extracted sample, visit https://issuetracker.google.com/issues/new?component=192708. (##) Suppressing You can suppress false positives using one of the following mechanisms: * Using a suppression annotation like this on the enclosing element: ```kt // Kotlin @Suppress("UnknownNullness") fun method() { problematicStatement() } ``` or ```java // Java @SuppressWarnings("UnknownNullness") void method() { problematicStatement(); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection UnknownNullness problematicStatement() ``` * Using a special `lint.xml` file in the source tree which turns off the check in that folder and any sub folder. A simple file might look like this: ```xml <?xml version="1.0" encoding="UTF-8"?> <lint> <issue id="UnknownNullness" severity="ignore" /> </lint> ``` Instead of `ignore` you can also change the severity here, for example from `error` to `warning`. You can find additional documentation on how to filter issues by path, regular expression and so on [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html). * In Gradle projects, using the DSL syntax to configure lint. For example, you can use something like ```gradle lintOptions { disable 'UnknownNullness' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore UnknownNullness ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).